Interest in renewable energy sources has been steadily increasing due to high oil prices, peak oil concerns, climate change worries, and government incentives. One area of particular interest is in developing liquid biofuels to replace gasoline, diesel, jet fuel, and the like, used in conventional internal combustion engines. Another area of interest is in developing biolubricants to replace petroleum-based lubricants.
The most common process used to produce liquid biofuels is based on the transesterification of vegetable oil or animal fat feedstocks. In these processes, alkyl alcohols react with long chain fatty acids to produce mono alkyl esters of the long chain fatty acids. Methyl esters are commonly known by acronym FAME, which stands for Fatty Acid Methyl Ester. Ethyl esters are commonly known by the acronym FAEE, which stands for Fatty Acid Ethyl Ester. Ester based biofuels are sometimes referred to as “first generation” biofuels.
More recently, hydrogenation processes have been developed that convert vegetable oil or animal fat feedstocks into a higher quality biofuel that is more like conventional petroleum-based fuels. The biofuel is produced through hydrotreating, which is the same process that is already used in today's petroleum refineries.
Hydrotreating entails direct hydrogenation of the feedstock fatty acids or triglycerides in the presence of a catalyst to produce the corresponding alkane. The hydrogen replaces other atoms such as sulfur, oxygen, and nitrogen and converts the lipid molecules into paraffins and isoparaffins. The result is a hydrocarbon fuel that contains very little oxygen or sulfur. These biofuels are sometimes referred to as “second generation” or “synthetic” biofuels. They may also be known as non-esterified renewable biofuels.
Biolubricants may be produced using a transesterification process with a longer chain alkyl alcohol such as octyl alcohol. The resulting ester has lubricating properties that are similar to petroleum-based lubricants.
Biofuels and biolubricants provide a number of advantages over conventional petroleum-based materials. One of the principal advantages of these biomaterials is that they are renewable. Biofuels and biolubricants are derived from natural sources that are capable of being renewed over a relatively short time period such as a growing cycle for plants or a lifecycle for microorganisms. In contrast, petroleum-based materials are formed through geologic processes that take millions of years to renew.
Biofuels and biolubricants do not lead to carbon dioxide accumulation in the atmosphere. Carbon dioxide is extracted from the atmosphere by the source organisms as they grow and is returned to the atmosphere when the biofuels and biolubricants are burned or decompose. Unlike petroleum-based fuels, the overall impact is carbon neutral.
Biofuels and biolubricants are easy to use and require little or no additional infrastructure investment. Most biofuels and biolubricants can be distributed using conventional petroleum tanks and pumps and used in existing engines. In most applications, biofuels are mixed with petroleum based fuels to combine the advantages of both. Engines that operate on pure biofuels may require some minor modifications.
Many biofuels also reduce harmful emissions from combustion engines. Ester based biofuels contain additional oxygen that makes the biofuel burn more completely. This reduces the emission of unburned hydrocarbons, carbon monoxide, and/or particulate matter. Non-esterified biofuels can greatly boost cetane levels in diesel fuel. They can also dramatically reduce tailpipe emissions from conventional diesel engines. Biofuels also contain very little, if any, sulfur, which reduces the emission of sulfur dioxide, a significant cause of acid rain. The end result is cleaner air and a cleaner environment.
Ester based biofuels and biolubricants are non-toxic and biodegradable. The fatty acid esters readily degrade in the environment in a relatively short period of time. Ester based materials are also much less toxic than conventional petroleum-based fuels. These properties make ester based biofuels and biolubricants especially suitable for use in environmentally sensitive areas.
Ester based biofuels have a higher flash point than petroleum fuels making them safer to store and transport. For example, the flash point of biodiesel is greater than 130° C., which is significantly higher than the 64° C. flash point of petroleum diesel. The risk of inadvertently igniting ester based biofuels is much smaller than for conventional petroleum products.
Non-esterified biofuels offer some different advantages. One of the biggest advantages is that the hydrogenation processes do not produce any non-fuel coproducts. Another advantage is that non-esterified biofuel has a high cetane number (85 to 99) and the cloud point can be adjusted from −5° C. to −30° C. It also does not experience any storage stability problems.
Although biofuels and biolubricants have shown tremendous promise, there are still a number of obstacles preventing them from being adopted on a wider scale. One of the biggest problems is that the biofuels are produced using resources such as land and water that compete with food production. This is especially a problem when it comes to biodiesel production. The preferred source for producing biodiesel is vegetable oil because of its low free fatty acid content. However, this source competes directly with the food supply for arable land and water resources. Other sources that have higher amounts of free fatty acids are less desirable because they require pretreatment before they can be processed using conventional methods.
Conventional biodiesel production is primarily accomplished using the transesterification process. In this process, oil feedstock containing less than 4 wt % free fatty acids are mixed with methanol or ethanol in the presence of a basic catalyst such as potassium hydroxide. The fatty material in the oil feedstock reacts with the alcohol to produce biodiesel. If the feedstock contains more than 4 wt % free fatty acids, then it must be pretreated in an acid esterification process.
The hydrogenation process used to produce non-esterified biofuels needs to be integrated with an oil refinery to avoid the need to construct a dedicated hydrogenation production unit. If it is not part of a refinery, the required hydrogen stream makes the process uneconomical. However, limiting the process to refineries reduces its usefulness and potential since the expense of transporting the raw feedstocks to these limited locations is significant.
Another problem associated with conventional hydrogenation processes is that they are typically integrated with a hydrotreater to make use of the hydrogen that is already used in the refinery to remove sulfur. However, this reduces the volume of conventional fuels that can be processed through the hydrotreater.
Conventional hydrogenation processes also require a catalyst to successfully produce non-esterified biofuels. Unfortunately, the catalyst adds additional cost and complexity to the system.